Method of and composition for dissolving metallic copper

ABSTRACT

A method of and composition for dissolving metallic copper and copper rich alloys are disclosed, wherein there is employed a peroxydiphosphate compound, preferably with an adjuvant copper acceptor or solubilizing agent, to provide a stable etchant system which operates well in either acid or alkaline medium.

United States Patent [191 Grunwald et al.

[ Dec. 18, 1973 METHOD OF AND COMPOSITION FOR DISSOLVING METALLIC COPPER Inventors: John J. Grunwald, New Haven; Leo J. Slominski, Bristol; Adela Landau, Watertown; Dilip G. Shah, Wolcott, all of Conn.

MacDermid Incorporated, Waterbury, Conn.

Filed: Apr. 21, 1972 Appl. No.: 246,151

Assignee:

US. Cl 156/18, 156/3, 252/792 Int. Cl. C23f l/00 Field of Search 156/3, 7, 8, l8,

[56] References Cited UNITED STATES PATENTS 3,634,262 1/l972 Grunwald et al. l56/l8 X 3,717,520 2/1973 Brindisi 156/18 Primary ExaminerWilliam A. Powell Att0rney-Merrill F. Steward et al.

[57] ABSTRACT 16 Claims, No Drawings METHOD OF AND COMPOSITION FOR DISSOLVING METALLIC COPPER I BACKGROUND OF THE INVENTION The invention relates to a method of and composition for effecting the controlled etching and dissolution of substantial amounts of a metal, more particularly copper and copper rich alloys such as brass and bronze. Such operations as chemical milling, stripping of cop per lamina (e'.g., plated deposits) from metallic or nonmetallic-substrates, and printed circuit manufacture, typify some of the more common applications of the invention. v

A variety of etchants is known for dissolving metal from the surface I of metal-base or composite nonmetallic substrate, metal-clad laminates. All known etchants are subject, to greater or lesser degree, to various objections. These include such matters as thermal instability of the etchant solution,catalytic decomposition in the presence of transition metal ions, low tolerance to change, of pH, low or uneven rate of etching, poor capacity for dissolved metal, generation of toxic vapors'during use, difficulties in shipping and/or in handling the materials in preparing the etchant solutions for use, and problems of spent etchant disposal.

Ferric chloride etchants for example, while used extensively and exhibiting excellent etching rates when freshly prepared, undergo rapid change of their etch rate during use. Also they are quite corrosive not only of copper but of ferrous metals as well; i.e., are not selective and thus require special handling techniques and equipment.

Similar difficulties are encountered with the concen trated mineral acid etchants such as hydrochloric, nitric, sulfuric and/or chromic.

Hydrogen peroxide is known to be a good oxidant and has frequently been tried for etching copper, but suffers from slow etch rate unless catalyzed by chloride, cuprous or other transition metal ions which are nevertheless prone to introduce unpredictability in the etch rate. Hydrogen peroxide is in any event highly unstable, particularly when thus catalyzed.

Chlorites in alkaline ammoniacal solution are successfully used on large commercial scale to dissolve copper; but the thermal stability of such systems is not as good as could be desired, and they catalytically decompose in the presence of common transition metal ions, such as iron and even copper itself, requiring frequent replacement of the solution. Dissolution of copper by chlorites alsoinvolves an exothermic reaction, and the heat generated during etching aggravates the decomposition problem further.

Persulfate compounds, although offering some advantages over other oxidants here mentioned, are thermally unstable and decompose catalytically in the presence of copper, which is further promoted by the exothermic nature of the etching reaction in such solutions. 7

Another problem which has caused difficulty in the practical utilization of these prior etchant systems is that they must generally be used as soon aspossible once they have been prepared, since the catalytic decomposition spoken of sets in and rapidly deteriorates their effectiveness. Consequently such prior etchant solutions are not very satisfactory for intermittent use over extended periods of time.

Attempts have been made to use different combinations of and supplements to the foregoing oxidants, but generally speaking while such combinations and/or supplements may improve certain aspects of a system, there usually occurs a concommitant worsening of a defect inherent in one or more of the individual components. Finally, most of the oxidants heretofore used to dissolve metals are not effective in both acid and alkaline media, thus requiring totally different oxidant systems where acid or alkaline conditions are required.

SUMMARY OF THE INVENTION It has now beenfound that unusually effective and stable metal etchant systems are provided by the use of peroxydiphosphate compounds. Systems in which such compounds are the sole oxidant are functional, but they are particularly effective when combined with a suitable chelating or complexing agent, or atleast with a solubilizing agent such as sulfuric acid. Furthermore, such etchant compositions are useful both in acid as well as in alkaline media, although in the latter case alkali metal hydroxides must be avoided unless accompanied by complexing or chelating agents, because use of the hydroxide alone produces a black adherent coating on cuprous metals in alkaline medium. See US. Pat. No. 3,657,023 (Ser. No. 48,766, filed May 15, 1970).

Peroxydiphosphates exhibit unusual stability for a peroxygen compound and are not catalytically decomposed in the presence of transition metal ions, unlike the persulfates which they otherwise resemble. Because of this stability, peroxydiphosphates can be used to etch or dissolve copper at elevated temperatures, thus enhancing their etch rate without sacrifice of etch bath life. Moreover, the peroxydiphosphate solutions exhibit unusual stability when used intermittently over extended periods of time.

lthas also been found that peroxydiphosphates provide a finer and more uniformly controllable etch on copper surfaces, producing a unique topography of etched surface that is especially beneficial for improving bond strength (i.e., adhesion) between laminated and plated copper, as where a copper clad epoxy substrate is overplated with copper in through-hole plating of printed circuit boards.

The tetrapotassium salt of peroxydiphosphoric acid is presently preferred as the principal oxidant, chiefly because of its greater availability; but the acid itself or any of its ammonium or alkali or alkaline earth metal salts having adequate solubility may be used in place of the potassium salt.

While as noted above, the oxidant alone in aqueous solution will etch copper, for practical purposes a copper ion acceptor compatible with the peroxydiphosphate is also employed. The term copper ion acceptor, or copper acceptor for abbreviation, is used herein to designate an agent or combination of agents which acts to hold the copper ion in soluble form in solution as the metallic copper is dissolved by the oxidant. Thus the term is used to include such solubilizing agents as the mineral acids, for example sulfuric, phosphoric or nitric, as well as organic acids such as acetic, hydroxyacetic, citric or gluconic. It also refers to chelating or complexing agents, more especially ammonium compounds and amines; Specific examples of the latter include ammonium hydroxide, the ammonium salts of the acids above mentioned,-both inorganic and organic.

' Ammonium chloride may also be included if used in alkaline medium but is undesirable under acid conditions due to possible formation of poisonous oxides of chlorine. In fact almost any soluble inorganic salt can be used if ammonium hydroxide is also present. Also the organic ammonia compounds are quite useful, such as the alkyl or alkylene primary and secondary amines and polyamines, more especially the methyl and ethyl members of this class.

Surprisingly, solutions of peroxydiphosphate containing only an alkali metal hydroxide, i.e., no copper acceptor, produce on copper a substantially black, adherent smut, and the presence of such alkali metal hydroxides is accordingly to be avoided for the purposes of this invention, as mentioned above, unless it is adequately complexed. This smut formation does not occur, even at high concentrations of the peroxydiphosphate salt, where it alone is present; nor of course in ammonium hydroxide solution where the ammonium ion acts as a complexer. For different reason, hydrochloric acid solutions of peroxydiphosphate should also be avoided because of the danger of chlorine dioxide formation.

Etchant compositions consisting of an admixture of a dry peroxydiphosphate salt and an ammonium salt, for example, provide substantial advantages from the standpoint of ease in preparing and shipping a readymade composition which is immediately usable simply upon addition to water, or to an aqueous solution of a suitable base or acid, such as ammonium hydroxide or sulfuric acid.

While the concentration of the peroxydiphosphate in the etchant composition can vary widely, the etch rates obtained will vary with its concentration, but not as a straight line function. In general, the effective oxidant concentrations are found to be within the range of 0.1 mole per liter to 1.0 mole per liter of etchant solution. While the lower limit mentioned represents about the minimum operative condition, the upper limit is not determined by operativeness but by solubility and economic factors.

The invention is illustrated by the following examples which are given by way of illustration only and are not intended to limit the operating procedures or materials employed in carrying out the invention beyond that indicated above and as defined in the accompanying claims.

EXAMPLE I Copper foil was immersed in an aqueous solution composed of 50 percent by volume of aqueous ammonia (28 percent ammonia by weight) (3.7 mole per 1iter) and 1 mole per liter (346 grams) of potassium peroxydiphosphate. Temperature was maintained at 120 F (49 C), and air was bubbled through to provide good agitation.

The rate of copperdissolution in this solution was found to be 0.138 mil/min.

EXAMPLE II The procedure of Example I was repeated under identical conditions, but in this case the concentration of the peroxydiphosphate was reduced to 0.5 mole per liter.

The rate of copper dissolution was found to be 0.226 mil/min.

EXAMPLE Ill Again the foregoing procedure was repeated under identical conditions except that the oxidant concentration was reduced to 0.3 mole/liter.

The rate of copper dissolution in this case was 0.145 mil/min.

The foregoing examples demonstrate that etch rates can be obtained which compare favorably under certain conditions with the etch rate of conventional ammoniacal chlorite etchants extensively used in commercial copper etching applications today. See U.S. Pat. No. 3,231,503, Example I, where copper was dissolved at a rate of 0.13 mil/min. at 25 C, without agitation. While the rate of dissolution for the ammoniacal chlorite etchant of the patent would temporarily be increased at the higher temperature employed in the foregoing example of this invention, and also of course if agitation were employed, operation of the chlorite system is not practical or economical under those conditions because of its relatively poor thermal stability, resulting in a rapid decomposition of the solution. By comparison, the foregoing etchant solutions of this invention can be operated quite successfully at the indicated higher temperature, and furthermore such solutions can be used intermittently; that is, allowed to cool down between periods of use and be reheated whenever operation is again desired.

EXAMPLE IV In this case, copper foil was dissolved in an aqueous solution containing 0.5 mole per liter of potassium peroxydiphosphate; 1.5 mole per liter (1 19 grams) ammonium bicarbonate; and 21 percent by volume aqueous ammonium hydroxide (approx. 1.55 mole per liter). Again the conditions of operation were solution temperature of 120 F and air agitation.

The rate of the etching of the copper was 0.286 mil/- min.

EXAMPLE V In this case copper foil was immersed in a solution containing 0.5 mole per liter of potassium peroxydiphosphate; 0.76 mole per liter (60 grams of ammonium bicarbonate); approx. 1.0 mole per liter (54 grams) of ammonium chloride; and 23.3 percent by volume of aqueous ammonium hydroxide (approx 1.72 mole per liter). Again the operating conditions were solution temperature of 120 F. and air agitation.

The rate of copper dissolution obtained in this example was 0.298 mil/min.

In general a range of 0.1 mole per liter to saturation of peroxydiphosphate and from about 3 to percent by volume of ammonium hydroxide (28 percent by weight) represent the operable limits, with the preferred ranges being about 0.3 to 1.0 mole of the oxidant and to 50 percent by volume of ammonium hydroxide.

The rates of metal dissolution in the foregoing Examples 4 and 5 compare quite favorably with rates reported in the aforesaid U.S. Pat. No. 3,231,503, as well as in the improved ammoniacal chlorite etchant solutions disclosed in US. Pat. No. 3,466,208. While the improved system disclosed in the later patent exhibits substantially better copper capacities than that reported in the earlier patent, the improved system too is subject to rapid decomposition once prepared. Due to the far greater thermal stability of the peroxydiphosphate etchant solutions of the invention, a much longer bath life is now realizable in a commercial etchant system.

EXAMPLE VI In this example, copper foil was immersed in a 0.5 mole per liter solution of potassium peroxydiphosphate containing percent by volume of concentrated sulfuric acid (66Be.) (approx. 1.8 mole per liter), sufficient to lower the pH to not over 3.5. The solution temperature was 120? F and air agitation was supplied.

The rate of copper dissolution obtained was 0.194 mil/min.

EXAMPLE VII Again the conditions were the same as in Example VI, except that the solution also contained approx. 0.45 mole per liter (60 grams) of ammonium sulfate Temperature and agitation conditions were maintained as before.

The etch rate obtained was substantially the same as in Example VI.

Similar results are obtained using other ammonium salts such as ammonium phosphate, ammonium nitrate and ammonium salts of organic anions; Similarly, other inorganic mineral acids such as phosphoric, chromic and nitric, or an acid salt such as sodium bisulfate, can be used with the oxidant.

In place of ammonium hydroxide in Examples I to III, a water soluble organic base can be employed. For example, methyl or ethyl primary and secondary amines and polyamines are useful.

Etchant solutions of the present invention prove unexpectedly good in pretreating copper surfaces which are subsequently plated with further copper by electroless and/or electrolytic processes. Such a procedure is used extensively in the production of electronic circuit boards where the starting laminates of copper foil and polymerized resin substrate are punched to provide through-holes, andthe punched boards then plated to effect electrical interconnection of laminate copper areas on opposite faces of the board by plating of the through-hole walls. One of the chief problems encountered in this operation is that of obtaining a firm bond between the initial laminate copper and the subsequently plated copper deposit. Failure to obtain good bonding between the two copper lamina results in poor electrical contact, either initially or subsequently due to ply separation brought about when the circuit board is subjected to thermal cycling, as occurs when it is dipped in a solder bath, for example. It has now been found that subjecting the initial copper clad resin laminate, subsequent to through-hole punching but prior to electroless copper deposition, to a relatively mild etch in a peroxydiphosphate solution materially improves the resulting bond between the laminate and plated copper lamina.

As an example of the presently preferred process of preparing printed circuit boards using the invention, the following cycle of operations is recommended:

l. The punched blank circuit board isimmersed for about 5 minutes in a mild alkaline cleaner to remove surface soil.

2. It is rinsed in cold water.

3. The board is etched for three minutes in one of the novel solutions described in the Examples above under the conditions there specified.

4. The etched board is again rinsed in cold water.

5. The board is next immersed for one minute in a hydrochloric acid solution (30 percent by volume).

6. Following this the board is immersed for five minutes in a commercial one-step catalytic metal electroless plating activator solution. A solution such as that described in Example I of U.S. Pat. No. 3,532,518 gives excellent results.

7. The board is rinsed thoroughly again in cold water.

8. It is next immersed for approximately one minute at room temperature in a mild acid accelerator, such as a dilute solution of fluoroboric acid.

9. Cold water rinse again.

10. The board is immersed in a commercial electroless copper solution to deposit copper on the through-hole walls and surfaces of the laminated copper foil. A typical commercial electroless copper plating solution such as Macuplex 9055 of MacDermid Incorporated is eminently satisfactory.

l 1. The board is rinsed in cold water.

12. Additional copper is deposited by immersing the board in a standard bright acid copper electrolytic plating bath to build up the final desired thickness of plated copper, ordinarily 3 to 5 mils.

13. The finished board is again rinsed in cold water and dried.

The foregoing examples of preferred peroxydiphosphate etchant compositions of-the invention, their concentrations and temperature conditions, are intended to be illustrative and it is to be understood that reasonable latitude of modification, substitu: tion and change in exact composition from the examples in a manner consistent with the spirit and scope of the disclosure is intended to be covered by the following claims.

What is claimed is:

1. An improved aqueous etchant solution for copper and copper rich alloys, said solution containing as solutes therein at least 0.1 mole per liter up to saturation of a peroxydiphosphate and a soluble copper acceptor in amount sufficient to keepdissolved metal in solution.

2. A solution as defined in claim 11, wherein said soluble copper acceptor is selected from the group consisting of aqueous ammonia, the ammonium salts of an organic acid, the ammonium 'salts of an inorganic acid other than hydrogen chloride, the alkyl and alkylene primary and secondary amines and polyamines having from 1 to 2 carbons in the alkyl group.

3. An improved copper etchant solution as defined in claim 1, wherein said peroxydiphosphate is present in amount of about 0.5 mole per liter.

' 4. An improved copper etchant solution as defined in claim 1, wherein said peroxydiphosphate is the tetrapotassium salt.

5 An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from .0.1 mole per liter to'saturation of peroxydiphosphate, and from at least 3 percent up to percent by volume of ammoium hydroxide (28 percent ammonia by weight).

6. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1.0 mole per liter of peroxydiphosphate,

and from about 20 to 50 percent by volume of ammonium hydroxide (28 percent ammonia by weight).

7. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1 .0 mole per liter of peroxydiphosphate and at least 0.5 mole per liter of an ammonium salt selected from the group consisting of the nitrate, sulfate and bicarbonate.

8. An improved copper etchant solution as defined in claim 1, wherein said soluble ammonium salt is the salt of an organic acid selected from the group consisting of gluconic, acetic, hydroxyacetic, and citric acids.

9. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1.0 mole liter of peroxydiphosphate and sufficient sulfuric acid to lower pH to not over 3.5.

10. The method of dissolving metallic copper and copper rich alloys, which comprises contacting the copper with an aqueous solution as defined in claim 1.

11. The method of dissolving metallic copper and copper rich alloys as defined in claim 10, wherein said 5 solution consists essentially of about 0.3 to 1.0 mole per liter of peroxydiphosphate and from about 20 to 50 percent by volume of ammonium hydroxide (28 percent ammonia by weight); maintaining said solution at a temperature between ambient room temperature and 150 F while said copper is contacted with said solution for from about seconds to 10 minutes depending on the temperature of the solution.

12. The method of dissolving metallic copper and copper rich alloys as defined in claim 11, wherein said solution also contains at least about 0.5 mole per liter of an ammonium ion supplied by an ammonium salt of an inorganic mineral acid.

13. The method of dissolving metallic copper and copper rich alloys as defined in claim 10, wherein said solution consists essentially of about 0.3 to 1.0 mole per liter of peroxydiphosphate and sufficient inorganic mineral acid to lower the pH to not over 3.5.

14. The method of dissolving metallic copper and copper rich alloys, which comprises contacting the copper with an aqueous solution as defined in claim 8.

15. In the method ofimproving the adhesion between a metallic copper or copper rich alloy substrate and a copper deposit plated thereon, where the copper substrate is first activated in a catalyst metal solution and then immersed in an electroless copper plating solution to form an overlying deposit of copper on said copper substrate, the step which comprises contacting the copper substrate, prior to activation thereof by said catalyst metal solution, with an aqueous etchant solution as defined in claim 1, maintaining said etchant solution at a temperature between ambient room temperature and 150 F, while immersing said copper substrate therein for from l5 seconds to 10 minutes, removing said substrate from said etchant solution and rinsing said substrate in water before proceeding to said activating and electroless plating steps.

16. A self-contained dry powder composition for use in acidic or basic aqueous solution to dissolve metallic copper and copper rich alloys, said composition consisting essentially of from 10 to 90 percent by weight of a water soluble peroxydiphosphate, and from 90 to 10 percent by weight of a water soluble copper acceptor selected from the group consisting of ammonium salts of inorganic acids, ammonium salts of organic acids, alkyl and alkylene primary and secondary amines and polyamines having up to two carbons in the alkyl group. 

2. A solution as defined in claim 1, wherein said soluble copper acceptor is selected from the group consisting of aqueous ammonia, the ammonium salts of an organic acid, the ammonium salts of an inorganic acid other than hydrogen chloride, the alkyl and alkylene primary and secondary amines and polyamines having from 1 to 2 carbons in the alkyl group.
 3. An improved copper etchant solution as defined in claim 1, wherein said peroxydiphosphate is present in amount of about 0.5 mole per liter.
 4. An improved copper etchant solution as defined in claim 1, wherein said peroxydiphosphate is the tetrapotassium salt.
 5. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.1 mole per liter to saturation of peroxydiphosphate, and from at least 3 percent up to 70 percent by volume of ammoium hydroxide (28 percent ammonia by weight).
 6. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1.0 mole per liter of peroxydiphosphate, and from about 20 to 50 percent by volume of ammonium hydroxide (28 percent ammonia by weight).
 7. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1.0 mole per liter of peroxydiphosphate and at least 0.5 mole per liter of an ammonium salt selected from the group consisting of the nitrate, sulfate and bicarbonate.
 8. An improved copper etchant solution as defined in claim 1, wherein said soluble ammonium salt is the salt of an organic acid selected from the group consisting of gluconic, acetic, hydroxyacetic, and citric acids.
 9. An improved copper etchant solution as defined in claim 1, which consists essentially apart from water of from 0.3 to 1.0 mole liter of peroxydiphosphate and sufficient sulfuric acid to lower pH to not over 3.5.
 10. The method of dissolving metallic copper and copper rich alloys, which comprises contacting the copper with an aqueous solution as defined in claim
 1. 11. The method of dissolving metallic copper and copper rich alloys as defined in claim 10, wherein said s solution consists essentially of about 0.3 to 1.0 mole per liter of peroxydiphosphate and from about 20 to 50 percent by volume of ammonium hydroxide (28 percent ammonia by weight); maintaining said solution at a temperature between ambient room temperature and 150* F while said copper is contacted with said solution for from about 15 seconds to 10 minutes depending on the temperature of the solution.
 12. The method of dissolving metallic copper and copper rich alloys as defined in claim 11, wherein said solution also contains at least about 0.5 mole per liter of an ammonium ion supplied by an ammonium salt of an inorganic mineral acid.
 13. The method of dissolving metallic copper and copper rich alloys as defined in claim 10, wherein said solution consists essentially of about 0.3 to 1.0 mole per liter of peroxydiphosphate and sufficient inorganic mineral acid to lower the pH to not over 3.5.
 14. The method of dissolving metallic copper and copper rich alloys, which comprises contacting the copper with an aqueous solution as defined in claim
 8. 15. In the method of improving the adhesion between a metallic copper or copper rich alloy substrate and a copper deposit plated thereon, where the copper substrate is first activated in a catalyst metal solution and then immersed in an electroless copper plating solution to form an overlying deposit of copper on said copper substrate, the step which comprises contacting the copper substrate, prior to activation thereof by said catalyst metal solution, with an aqueous etchant solution as defined in claim 1, maintaining said etchant solution at a temperature between ambient room temperature and 150* F, while immersing said copper substrate therein for from 15 seconds to 10 minutes, removing said substrate from said etchant solution and rinsing said substrate in water before proceeding to said activating and electroless plating steps.
 16. A self-contained dry powder composition for use in acidic or basic aqueous solution to dissolve metallic copper and copper rich alloys, said composition consisting essentially of from 10 to 90 percent by weight of a water soluble peroxydiphosphate, and from 90 to 10 percent by weight of a water soluble copper acceptor selected from the group consisting of ammonium salts of inorganic acids, ammonium salts of organic acids, alkyl and alkylene primary and secondary amines and polyamiNes having up to two carbons in the alkyl group. 